This invention relates to semiconductor devices with high electron mobility, more particularly to FETs utilizing electrons accumulated in the neighborhood of a single heterojunction due to the difference in electron affinity between the two different kinds of semiconductors which form a single heterojunction. More specifically, this invention relates to active semiconductor devices each of which has a single heterojunction formed between a pair of layers fabricated with two different semiconductors having different electron affinities from each other and which employs a field effect caused by one or more gates for regulation of the concentration of electrons accumulated along the single heterojunction due to the difference in electron affinity, resulting in the impedance of a channel formed with the accumulated electrons between an input and an output terminal being regulated depending on the voltage applied to the one or more gates.
The field effect transistors available in the prior art are classified into three types, including the junction gate type, the insulated gate type and the Schottky barrier type. Out of these three families, the insulated gate type and the Schottky barrier type (Metal semiconductor or MES) are rather easy to produce in the form of integrated circuits. Therefore, insofar as the integrated circuits are concerned, these two types are predominantly employed. For the purpose of improving the switching speed of the FETs, various means including decrease of geometrical dimensions are employed. However, improvement in switching speed is inherently limited by electron mobility or the speed of electrons moving in a conductive channel. In other words, improvement in electron mobility is the easiest means or even the essential means for improvement of the switching speed of a FET. It was believed, however, that electron mobility is determined by the kind of and the concentration of impurities doped into a semiconductor, temperature, etc., and that there is a limitation for improvement of electron mobility.
It is noted, however, that R. Dingle et al. have disclosed results of efforts for improvement of electron mobility which were successfully realized by a multi-layered structure of semiconductors including plural heterojunctions. Their report entitled "Electron Mobilities in Modulation-doped Semiconductor Heterojunction Superlattices" disclosed in Applied Physics Letters, Vol. 33, Pages 665 through 667 on Oct. 1, 1978 reveals that albeit the electron mobility of GaAs doped with n-type impurities at a concentration of 10.sup.17 /cm.sup.3 is approximately 5,000 cm.sup.2 /V.sec at the temperature of 300.degree. K., a multilayered structure fabricated by alternately growing n-doped AlGaAs layers and undoped or unintentionally doped GaAs layers allows the GaAs layers to have an electron mobility of approximately 20,000 cm.sup.2 /V.sec at the temperature of 77.degree. K. This improvement in electron mobility was realized in the electrons accumulated in the GaAs layer contiguous with the heterojunction due to the difference in electron affinity, because of the lesser ionized-impurity scattering in the undoped or unintentionally doped GaAs layer at a cryogenic temperature